This invention relates to devices for inspection and measurement of pipe threads and, more particularly, to the inspection of pipe threads utilized in oil-drilling equipment to determine that such threads are within tolerances.
Threaded couplings and the threaded end-portions of pipes are used for joining pipes together in numerous situations ranging from applications in the home to industrial applications. In certain situations, particularly in the case of oil-drilling equipment, pipes having a relatively large diameter and long lengths are employed. Such pipes require a correspondingly large area of thread to ensure adequate strength to a threaded joint. To ensure ease of joining such pipes, and to insure that the resulting joints will have adequate strength and will not leak, it is essential that the threads be made with sufficient precision to allow the threaded end-portion of a pipe to be properly threaded into a coupling.
A problem arises with respect to the inspection process for determining that adequate precision is present in the pipe thread, namely, that the process is not accomplished as readily as would be desired and that, furthermore, not all measurement parameters can be inspected by existing inspection equipment. The parameters of a thread which are useful in determining the accuracy of fit in a joint are discussed in the American Petroleum Institute Specification 5B. These include lead error, thread height error, pitch line non-linearity, average taper and interval taper. Reference is made to pp 2 & 14 of the API Specification for a discussion of the latter two parameters. The problem is compounded in that, heretofore, more than one article of measuring equipment needed to be employed in the measurement of these parameters while no commercially available equipment appears to be available for measurement of the pitch line non-linearity.
The following U.S. Patents are exemplary of the state of the art. A portable thread measuring device for measuring the distance of a pipe thread to the end of the pipe is taught in the Horton U.S. Pat. No. 4,184,265. Devices for measuring taper are shown in Dietrich U.S. Pat. No. 3,861,047, Mittenbers U.S. Pat. No. 3,047,960 and Possati U.S. Pat. No. 4,139,947. A device for measurement of pitch diameter of a thread is taught in Kaifesh U.S. Pat. No. 3,827,154, Peterson U.S. Pat. No. 2,937,458, Schasteen U.S. Pat. No. 4,202,109 and Heslin U.S. Pat. No. 3,271,872. A device for gauging the shape of threads is taught in Johnson U.S. Pat. No. 3,318,011. Devices for measuring thread diameter are taught in Reish U.S. Pat. No. 3,432,935.